Consequences of global warming on ocean circulation

Global warming is likely to have a number of effects on the ocean. We know that carbon dioxide dissolves more easily in cold water than in warm water so warmer temperatures will reduce the ability of the oceans to take up carbon dioxide and this will further enhance the greenhouse effect. Higher temperatures are also predicted to increase the input of freshwater into the high latitude oceans. Computer models suggest that this additional freshwater comes from increased rain at mid and high latitudes and from the melting of ice sheets. Warmer temperatures also cause expansion of water and, along with the additional water from ice melt, will result in a rise in sea level and may cause flooding.

Ocean circulation is very sensitive to the amount of freshwater entering the system. Freshwater controls the density of seawater and therefore the ability of seawater to sink when it is cooled. If the water is too fresh, cooling won't make it dense enough to sink into the deep ocean. If water doesn't sink at high latitudes there is only wind driven forcing for the Gulf Stream and therefore reduced water circulation around the oceans.

We have some geological evidence that the thermohaline circulation has shut down in the past. Warming at the end of the last ice age ~15,000 years ago melted the ice sheets over North America and increased the freshwater input to the North Atlantic. This reduced the saltiness of seawater, preventing it sinking, and therefore decreased deep water formation. There is evidence that this caused the shutdown of the thermohaline circulation, caused the Gulf Stream to move south and reduced heat transport to Northern Europe. This interupted the warming seen at the end of the last Ice Age. Ice core and deep sea sediment records show that temperatures in north west Europe fell by 5 oC in just a few decades returning the North Atlantic region to Ice Age conditions. This era of the Earth's history is known as the Younger Dryas Cold Period. We also have some evidence that ocean circulation is changing now as a result of global warming. Observations show that the North Atlantic has become steadily less salty over the past 40 years.

1. Around 13,000 years ago the last Ice Age ended and the Earth became warmer. This warming increased the amounts of freshwater entering the North Atlantic Ocean as the ice sheets melted. Ocean circulation shut down and the North Atlantic region became colder again. Freshwater inputs stopped once all the ice had melted and ocean circulation was able to start up again. This caused a rapid (on geological timescales!) increase in temperature as warm water from the tropics was able to flow again into the North Atlantic. Figure courtesy of R. Alley and the CLIVAR project.

Trying to predict the impact of global warming requires use of complex computer models which all include assumptions about future conditions that cannot be tested properly. As a result, different models predict differing outcomes of global warming. Some models suggest that global warming will lead to a weakening of the thermohaline circulation during this century but that ocean circulation will not shut down completely, others show a complete breakdown of ocean circulation as a result of increased inputs of freshwater. It is difficult, therefore, to estimate how our climate would be if ocean circulation changed.

2. Nine different computer models all predict very different changes to ocean circulation as a result of global warming. Some models suggest little or no change in the amount of deep water formed in the North Atlantic and therefore little change to the thermohaline circulation. Others, however, suggest that global warming will reduce deep water formation by a huge amount. The unit used is the Sverdrup (Sv) which gives the volume of water moving over time. 1 Sv is a million cubic meters of water moving every second! The worst case scenario is that global warming will reduce the flow of water from the surface to the deep in the North Atlantic by 15 Sv by the year 2100, greatly reducing the thermohaline circulation. Figure courtesy of the CLIVAR Project.

Complex models suggest a cooling of around 2 oC over most of Europe as a result of the reduced transport of heat from the Caribbean as the Gulf Stream weakens.

3. This figure shows how air temperatures are predicted to change if the thermohaline circulation simply stopped. Most computer models suggest that temperatures in Europe would become colder as the Gulf Stream slowed down and less heat was brought from the Tropics to northern Europe. Courtesy of Michael Vellinga and Richard Wood and the CLIVAR Project.

The impact of greenhouse gas induced warming has to be superimposed on this to give an overall prediction of climate. Over the world as a whole, temperatures are predicted to increase. Over Europe, however, most models suggest that temperatures would change little or that there would be a slight warming if the thermohaline circulation collapsed. Along with this slight warming, the climate is suggested to become wetter and more stormy. Predicting where and when these storms occur is really difficult. So we still need better observations and more realistic computer models to reduce our uncertainty about what will happen to our climate in the future.